Ganesh Kumar, Scientist, Central Building Research Institute, for providing their valuable inputs for the improvement of the data taken from available field research. The magnitude of available vacuum pressure in the field was predicted by several numerical simulations.
Preamble
Preloading
Surcharge Preloading
Vacuum Preloading
Vacuum Consolidation System and Construction
The system mainly consists of a system of vertical drains connected to horizontal drains as part of surface drains at the top to carry the discharged water away from the treatment area. Vertical drains are an important part of the vacuum consolidation system, as these are the medium for transferring vacuum pressure to the treatment area.
Mechanism of Vacuum Preloading
Another technique is that of the Menard system, in which a 1.5 meter thick primary fill is placed directly on the top of the sand mat under the membrane to increase the stability and sealing of the system.
Modelling of Vacuum Preloading
Therefore, this study using 2D analysis finds that an equivalent plane strain analysis is sufficient from a computational point of view, in the case of multi-drain analysis and large infrastructure projects.
Objective and Scope of the Study
Organization of the Thesis
A unique design chart has been developed to determine the degree of consolidation when vacuum preloading is adopted for any type of soft soil condition.
Introduction
Vertical Drains
Factors affecting drain performance
Vacuum Consolidation
Since then, the method has gained immense popularity due to its advantages over conventional additive loading. With the development of technology and the manufacture of high-quality drains, pumps and airtight plates, the method overcame the initial obstacles of maintaining effective vacuum pressure in the ground.
Experimental Studies on Vacuum Preloading
In laboratory tests, it was found that vacuum-filled samples showed less permeability anisotropy than back-filled samples. Scanning electron microscope (SEM) images showed that at shallow depths, the vacuum-preloaded samples showed more flaky structures, while the additionally loaded samples showed a dispersed distribution.
Numerical Studies on Vacuum Preloading
It was also observed that well resistance is only effective enough for PVDs with deeper penetration depth. In addition, Indraratna et al. presented a method to transform horizontal permeability from axisymmetric state to plane strain state using a modified consolidation theory for vacuum preloading (Figure 2.5). For closer spacing between PVDs (i.e., 1.0 m), it was observed that predictions with constant vacuum pressure above ground and linearly varying vacuum pressure along the drain were consistent with the field results.
It was determined that equivalent plane strain method is adequate for multi-drain analysis using additive or vacuum prestressing conducted a numerical study of a port reclamation project in Vietnam using PLAXIS 2D under fully coupled flow deformation framework.
Case Studies on Vacuum Preloading
presented a 2D and 3D multi-drain analysis in combination with Biot's consolidation theory for vacuum consolidation modeling using FEM software ABAQUS. Tang and Shang presented a case study on vacuum consolidation for the construction of Yaoqiang airport runway. It was observed that a combined vacuum supplement accelerates radial consolidation and controls lateral displacement. This is a case study of India's first vacuum consolidation trial at Kakinada Port, Andhra Pradesh, India.
Herve presented a case study on the construction of deep water ports along the Cai Mep River in Vietnam.
Summary
Also, several layers of sand may be present in the treatment soil at some depth, which leads to a decrease in vacuum pressure. The loading sequence was introduced by calculating the increase in undrained shear strength at each stage. The time taken to reach the required vacuum pressure is also not generally considered in the analysis.
In the following chapters, the influence of vacuum pressure on the consolidation properties of soft clays is studied using the PLAXIS 2D numerical model.
Introduction
Finite Element Method (FEM)
PLAXIS 2D Software
Fully Coupled Flow Deformation Analysis
Steady-state pore pressures are calculated at the end of the calculation phase based on hydraulic conditions. The fully coupled flow deformation analysis takes into account the unsaturated behavior of the soil, where necessary, and includes suction in the unsaturated zone above the water table. Instead of excess pore pressure, the method operates on total pore pressure which is the sum of steady state and excess pore pressure.
3.4) Where σ = total load, σ' = effective load, pw = pore water pressure, ν = pore fluid velocity gradient, = volumetric strain rate, K = permeability matrix, h = pore fluid potential energy height, x = direction of gravity and xD = height of reference plane.
Elements used in the study
15-node triangular elements: 15-node triangular elements (Figure 3.1) belong to the category of area elements, used to model the surface and soil mass under plane stress conditions. These elements provide fourth-order interpolation for displacements and twelve stress points (Gaussian) for stress calculations. In FEM formulations, local coordinates ξ and η are available for triangular elements; In addition, the auxiliary coordinate ζ = 1 – ξ – η is also added for better interpolation (the local numbering and position of the nodes is shown in figure 3.2).
The excess pore water pressure along the drain is zero allowing the water to exit the model at atmospheric pressure.
Load Stepping Procedure
The accuracy increases as the time step decreases; however, for consolidation problems, there exists a threshold below which a rapid decrease in accuracy is observed. Where Cv is the consolidation coefficient, H is the height of the element, α is the time integration coefficient and η is a constant parameter depending on the type of element.
Material Models
Various material parameters include modified compression and swelling indices, as well as Mohr-Coulomb model failure parameters. As illustrated in Figure 3.4, cohesion results in an elastic region that lies partially in the tension zone. Friction angle: It is recommended to use critical state friction angle, based on small deformations.
Contribution of Poisson's ratio is not significant for primary load problems; however, it becomes important in reading conditions.
Boundary Convergence study
Boundary Conditions
Mesh sensitivity study
Vacuum Consolidation in PLAXIS 2D
This negative groundwater level must not exceed 10 m below the GWT, which is equivalent to an atmospheric pressure of 100 kPa. A reduction in the groundwater level in the drains will lower the groundwater level in the entire area, which would cause the soil to become unsaturated, which would result in a change in the permeability of the soil mass. However, since in reality the global water table is not falling, this change in permeability is unrealistic and should not occur.
In addition, to avoid reduced permeability, the hydraulic model is set to saturated in the groundwater flow data sheet.
Validation with Case Study of Reclamation Project in Vietnam
Introduction
Numerical model
As predicted in the study, settlements are very high and this reinforces the need for land improvement. It was found that the data were in good agreement with the already published result.
Summary
Introduction
Field study at Kakinada Port
The soil properties obtained from the laboratory tests (Table 5.2) together with the assumed properties were used to model the behavior of the test section. A fully coupled flow deformation is performed when it is necessary to analyze the simultaneous development of deformations and pores in saturated and partially saturated soils due to time-dependent changes of the hydraulic boundary conditions. Boundary convergence study was performed to determine the extent of the horizontal boundaries of the model (Figure 4.2).
The vacuum pressure is determined by the negative head of water along the length of the drains (a reduction in groundwater head of 10 m means under pressure of 100 kN/m2, i.e. full vacuum).
Results and Discussions
It can be seen from Figure 4.4 that numerical results are not consistent with field results. Pore pressure reduction in the middle of the clay layer depth for Case 4 (Figure 4.5) as well as without any vacuum pressure (VP) loss is shown in Figure 4.8. When the vacuum is turned off, the pore pressure does not reach the initial value immediately throughout the treatment area (Figure 4.10(a) & 4.10(b)).
The results show that settlements obtained by all three cases reach the settlement given by single stage application of 85 kPa vacuum pressure (Figure 4.13).
Parametric Studies
The dimensionless parameter Settlement Ratio was introduced and for 300 days of load operation, its magnitude decreased from 0.15 at the beginning to 0.11 towards the end (Figure 4.17). Effect of Poisson's Ratio on Pore Pressure Reduction and the Mandel-Cryer Effect Section 4.4.2 stated that there is a temporary increase in pore water pressure before it begins to decrease, and this effect is called the Mandel-Cryer effect. The increase in pressure starts near the drainage boundary and is then slowly transmitted into the interior of the soil mass.
The magnitude of the Mandel-Cryer effect also depends on Poisson's ratio (ν'), as shown in Figure 4.18, it decreases with increasing Poisson's ratio.
Summary
The maximum vacuum pressure developed in the field is estimated to be 30-35 kPa and generally 25-30 kPa. The magnitude of the Mandel-Cryer effect was observed to decrease with increasing Poisson's ratio. The application of the additional load after the activation of the vacuum load was found to be adequate to avoid sudden failure of the bearings when compared to the additional load before the vacuum load.
Soil improvement was evaluated by settlement characteristics and settlement was found to be reduced by 85-90% after treatment.
Introduction
Differential Equation for Radial Consolidation
The differential equation governing the unsteady one-dimensional radial distribution of pore pressure through a vertical drain is given as. Where, t is the elapsed time after applying the vacuum pressure, r is the radial distance from the PVD centerline, u is the excess pore pressure created due to the application of the external load, and Cr is the radial consolidation coefficient.
Nature of the Radial Consolidation Equation
Boundary and Initial Conditions
Whereas, radial terms will require two boundary conditions, from one to two domain boundaries. In boundary condition 4a, it is assumed that the vacuum pressure – pvac is fully developed inside the well, i.e.
Finite Difference Method
Difference quotients using Taylor series
Formulation of Finite Difference
Algorithm
5.16) Equation 16 is a finite difference scheme for calculating the vacuum pressure developed at any point of a PVD unit cell under the influence of vacuum pressure along the PVD.
Results and Discussion
Design Chart for Radial Consolidation under Vacuum Preload
The following steps should be followed to find the time required to achieve the desired degree of consolidation.
Application to case histories
From the field measurements, the DOC was estimated to be 95%, from Figure 5.12, the DOC at 90 days was estimated to be 90%, which differs from the field result by 5%, but is sufficiently close. Soil layers at Tianjin port were found to be in four layers [22], of which the first and third layers were found to be similar and represented the greatest depth of the soil; and therefore the properties corresponding to these layers were adopted for the rest of the layers as shown in Table 5.3. DOC shape field measurements were estimated to be 80% (using pore pressure profile as discussed in Section 4.4.2).
However, analyzes of homogeneous soil layers can be performed with a reasonable degree of accuracy.
Summary
Soft soil model (based on Cam-Clay model) was considered to simulate the behavior of marine clay and linear drainage elements are included for PVDs. The numerical model was validated with available case study at a port reclamation project in Vietnam and the results were found to be in good agreement with the published results. Along similar lines, a field study at Kakinada Port, India was modeled and the behavior of soft soil was analyzed under the influence of vacuum pressure.
Parametric studies performed on soft soil revealed that the change in pore water pressure depends on Poisson's ratio at the initial stage of vacuum pressure application.
Conclusions
Study of Vacuum Consolidation in Kakinada Coastal Clay Deposits
Analysis of Radial Consolidation under Vacuum Preloading
Scope of Future work
![Figure 1.2: Comparison of lateral deformation in soil due to surcharge and vacuum preload [2]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10530249.0/17.892.158.837.368.612/figure-comparison-lateral-deformation-soil-surcharge-vacuum-preload.webp)
![Figure 2.5: Conversion of axisymmetric unit cell into plane strain wall: (a) axisymmetric; and (b) plane strain [20]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10530249.0/29.892.237.657.131.512/figure-conversion-axisymmetric-plane-strain-axisymmetric-plane-strain.webp)
![Figure 2.6: Pore water pressure distribution verses depth under combined surcharge and vacuum load [4]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10530249.0/31.892.245.638.265.675/figure-pore-pressure-distribution-verses-combined-surcharge-vacuum.webp)
